Currently the Third Generation Partnership Project (3GPP) is evaluating the potential benefits of uplink transmit diversity in the context of High-Speed Uplink Packet Access (HSUPA), see 3GPP Tdoc RP-090987. With uplink transmit diversity, User Equipments (UEs) that are equipped with two or more transmit antennas are capable of utilizing all of them. This is achieved by multiplying the signal s(t) with a set of complex weights wi; see FIG. 1. Note that i=1 . . . N where N denotes the number of transmit antennas. The rationale behind uplink transmit diversity is to adapt the weights so that the user and network performance is maximized. Depending on UE implementation, the antenna weights may be associated with different constraints. Within 3GPP two classes are considered:
Switched antenna diversity, where the UE at any given time-instance transmits from one of the antennas only. Thus if w1≠0, then wi=0 for all j≠i.
Beam forming, where the UE at a given time-instance can transmit from more than one antenna simultaneously.
While switched antenna diversity is possible for UE implementations with a single power amplifier (PA) the beam forming solutions may require one PA for each transmit antenna. I.e., beam forming requires one PA for each active antenna.
Irrespective of the considered class, the selection of the antenna weights can be based on that:
The Node-B provides explicit feedback to the UE stating which weights wii=1 . . . N that the UE should use when transmitting the signal. This would require that a new feedback channel is introduced; or
The UE autonomously decides the antenna weights. To select the weights wi1=1 . . . N the UE may monitor existing feedback channels (that are transmitted for other purposes) such as Fractional-Dedicated Physical Channel (F-DPCH) or Enhanced Dedicated Transport Channel Hybrid Automatic Repeat Request Acknowledgement Indicator Channel (E-HICH).
As mentioned above, the fundamental idea behind uplink transmit diversity is to exploit the variations in the effective channel. The term effective channel here incorporates the effects of the transmit antenna(s), transmit antenna weights, the receiving antenna(s), as well as the wireless channel between transmitting and receiving antennas. Finally, note that both switched antenna diversity and beam forming transmit diversity may result in that the antenna weights are changed abruptly, e.g., if a UE that applies switched antenna diversity changes antenna and starts transmitting on antenna 2 instead of antenna 1 then w1/w2 will change from 1/0 to 0/1. A consequence of this is that the effective channel as perceived by the receiving Node-B may change abruptly.
A Node-B receiver uses channel estimates based on a filtered version of the instantaneous channel, as estimated by the Node-B. Also, the interference estimate used, e.g., in the inner loop power control (ILPC) is based on a filtered version of the instantaneous interference measured by the Node-B.
Hence, whenever the UE changes its antenna weights—and especially if the change is abrupt such as in switched antenna diversity, or with significant changes to the weights in beam forming—the channel estimates used by the receiver will become inaccurate. This reduces the receiver performance. A UE change in antenna weights will also result in that the interference caused by this UE is changed abruptly. This, sometimes referred to as flash light effects, will be particularly true in situations where the interference is on a cell edge where it is likely to be a dominating interference to another UE.
One may realize that situations where the UEs constantly change their antenna weights abruptly may result in reduced overall performance. This is among other things a consequence of the uncoordinated UE behavior, which may result in increased interference variations. The potential gain arising associated with uplink transmit diversity schemes will also depend on the channel's coherence time.
Another final implication of uplink transmit diversity is that there will be a discontinuity in the power received by the Node-B whenever the effective channel between the UE and Node-B changes. The discontinuity will be a combined effect of that:
The wireless channels between the two or more transmit antennas and the receiving antenna(s) are different, and
The antenna gains of the two or more transmit antennas are different.
As there is a discontinuity in the received power, the performance of the power control may be affected. A discontinuity in the received power may lead to a transient time period, or convergence time, until the outer loop power control (OLPC) and ILPC have settled. The transient time period may cause supplementary interference variation, having a negative impact on the system. Also, if the UE switches to an antenna with less favorable conditions, or if the channel estimates become so inaccurate that Node-B fails to demodulate the data, this may result in that the OLPC Signal-to-Interference Ratio (SIR) target is increased even if the link quality has improved.